The most widespread medium for distributing motion pictures is the videocassette. However, digitally encoded optical disks are in theory far superior for the distribution of motions pictures and other forms of presentation. Especially advantageous is the use of "compressed video," by which it is possible to digitally encode a motion picture on a disk no larger that the present-day audio CD. Information storage on a higher density optical disk could allow a single disk to contain multiple versions (e.g., R-rated and PG-rated) of the same motion picture, multiple soundtracks in different languages, and data for other related or even unrelated signals.
Because of the different television industry standards used throughout the world, there are an equal number of videocassette standards. An NTSC videotape sold in the United States, for example, will not play on most videocassette players to be found in England. In order for a software publisher not to have to produce optical disks in as many different formats as are presently required for videocassettes, it would be far preferable for the data stored on an optical disk to be converted to a particular standard by the player. That way, the same disk could be sold anywhere in the world. Because of the advantages offered by the storage of digital data on optical disks, it is possible to achieve this flexibility along with the storage of so many different signals.
Despite the fact that optical disks afford tremendous bit densities, there are nevertheless limits to how much information (data) can be stored on an optical disk. Especially if multiple versions of a motion picture, multiple soundtracks, and other kinds of signals are to be reproduced from the disk, care must be taken in how the data is organized on the disk. Prior art approaches to disk data organization have been ill conceived. Considerable work has been done on developing compressed video standards, especially the MPEG1 and MPEG2 standards, and one would have thought that the full bit savings offered by these standards would have made their way to optical disks. But that is not the case. The approach being voiced by developers of optical disk products is one which establishes an average bit rate for a motion picture, based on the compression afforded by the new standards, and use of that bit rate to represent the entire motion picture. This is not to say that every represented frame requires the same number of bits. Clearly, an impetus for video compression is the fact that different frames may be represented by different numbers of bits; relatively few bits are required in going from one frame to the next which is practically the same, while numerous bits are required to represent the first frame of a new scene. Nevertheless, system designers envisage that the number of bits per unit of time will be constant for the entire duration of a motion picture. Standard buffering techniques are contemplated for storing a sufficient number of bits to represent a succession of frames, with just those bits required for each frame being accessed at the frame rate. The basic shortcoming of this approach is that it assumes that the same average bit rate is applicable to an entire 2-hour movie, when that may not be the case at all. For example, a car-chase scene that lasts for 15 minutes may require a very high bit rate, while other scenes in the same motion picture may require a very low bit rate over an extended period of time. To apply a single average to the entire motion picture results in the needless storage of bits for slow-changing scenes, and an insufficient number of bits to represent fast-changing scenes.
What further complicates matters is that even if a variable bit rate is employed, a rate which changes to reflect how much video information must be represented (as opposed to limiting the amount of information that can be represented with a fixed bit rate), the other signals stored on the disk may similarly be represented with maximum efficiency by employing variable bit rates, but bit rates which vary differently from that required for the video. For example, a fast-changing scene with no sound requires a high video bit rate but a low soundtrack bit rate. To maximize the amount of information which can be stored on a disk, it is not only necessary to employ a variable bit rate, but to employ variable bit rates for the different signals to be represented, with the several bit rates changing in accordance with the needs of the respective signals that they represent rather than being keyed to each other.
It is a general object of our invention to provide a data storage format for a software carrier that permits efficient use of the bit capacity of the carrier.
It must be understood that the principles of the present invention are not limited to any particular types of carriers or any particular kinds of software, although there is no question that the invention has particular application to optical disks. Nevertheless, it is to be understood that the invention is not limited to a particular medium (for example, it is applicable to tape carriers and all digital storage media), nor is it limited to just the distribution of motion pictures. For example, in an extreme case, the invention is applicable to the distribution of a library of still pictures, in which there is no "motion" at all. The term "software publisher" thus embraces much more than a motion picture company, and the term "carrier" embraces much more than a digitally encoded optical disk.